Do I need a vapor barrier with spray foam?

Closed-cell spray foam at 1.5" or thicker is already a Class II vapor retarder, so it does not need a separate vapor barrier; open-cell foam in Climate Zone 6 and 7 does need one on the warm side.

The rule is simple but often misapplied. Vapor barriers slow moisture movement from the warm interior toward cold surfaces in winter. Closed-cell foam does that job itself once it is thick enough. Adding a polyethylene sheet behind or in front of closed-cell can trap moisture between two impermeable layers and cause rot or mold.

• Closed-cell ≥ 1.5": no separate vapor barrier needed. It is a Class II vapor retarder.
• Open-cell in Zone 6/7: vapor-retarder paint or a Class II smart membrane on the interior face is required.
• Below-grade assemblies: closed-cell directly to concrete, no poly behind it.
• Never: poly on both sides of an assembly.
• Unvented roof decks: closed-cell is the safer choice because it keeps the condensing surface warm and retards vapor.
• Code check: always verify with the local AHJ, especially in Climate Zone 7 where requirements can be stricter.

The trade-off is forgiveness. Closed-cell is more expensive but forgiving because it handles both insulation and vapor control. Open-cell is cheaper and vapor-open, but it shifts the vapor-control burden to another layer that must be installed correctly.

Confirm the foam type and thickness in writing before installation, and ask the contractor to point out where the vapor-control layer is in each assembly.